Optical navigation devices use a light source to illuminate a navigation surface so that an optical imager can generate digital images for computing motion. For example, an optical imager can image a finger on a surface and generate a navigation signal based on comparisons of sequential images.
In conventional optical imagers, the exposure time that the optical imager is sensitive to light is controlled by a shutter controller. Like a physical shutter, an electronic shutter controls the time that light is allowed to integrate on photodiodes within the optical imager. The shutter duration, or time that the optical imager integrates the light, can depend on the amount of internal light (e.g., when a finger is placed on the optical imager) and the amount of ambient light (e.g., when the finger is not placed on the optical imager). Some conventional imagers use fixed values for the shutter durations. Other conventional imagers use variable shutter values which continuously adjust to optimize the navigation signals. Variable shutter values can adjust continuously to changing levels of ambient light that exist around the optical imager. However, the variability of the shutter values can make it difficult to detect a transition between navigations states, for example, whether or not a finger is present on the optical imager. Additionally, the changing shutter values can cause erroneous movement detection based on changes in ambient light conditions, even when there is no finger present on the optical imager. The erroneous movement detection is then translated into erroneous navigation signals which are manifested, for example, by erratic cursor movements or content scrolling.
Conventional imagers also use fixed shutter threshold values, which regulate the amount of noise produced during a transition from a finger-on state to a finger-off state, or vice versa. Hence, when the shutter values, or durations, change by more than the fixed shutter thresholds, the optical imager can transition between the finger-on and finger-off states, while limiting the amount of noise that might result, for example, from ambient light starting to enter the optical imager before the optical imager transitions from the finger-on state to the finger-off state. However, as the ambient light levels and the respective shutter values vary, the fixed shutter threshold values may be ineffective. For example, the ambient light conditions may be lighter or darker than the internal light conditions, and the fixed shutter threshold values do not necessarily provide adequate noise regulation in both scenarios.